1. Field of the Invention
This invention relates to a manufacturing process for a semiconductor device and a low-pressure chemical vapor deposition reactor utilizing the process.
2. Description of the Related Art
As a semiconductor device has been increasingly integrated, a two-dimensional design rule such as a circuit pattern has recently become finer and smaller. In particular, a two-dimensional area in wiring has been reduced to increase of a wire resistance, while a semiconductor device has come to be of higher speed so that needed operating characteristics for the device requires keeping the wire resistance equal to or lower than that before size reduction.
To solve the problem in a wire resistance, a variety of materials for a wire have been investigated and practically used. Conventionally, an electrode such as a gate electrode for a general CMOS semiconductor device and a capacity-section electrode for a DRAM (random access memory) has been formed by diffusing phosphorus, in a high-temperature diffusion furnace, on polycrystal silicon film (Poly-Si film) deposited using a low-pressure chemical vapor deposition method (LPCVD method) to diffuse a dopant into the film for reducing an electrical resistance. The phosphorus diffusion method diffuses a dopant in a film surface. It has been, therefore, difficult to diffuse the dopant completely from the surface to a bottom of a minute via. In addition, for a low concentration of dopant, a dopant level cannot be well controlled to be even in the depth direction of the Poly-Si film.
Recently a phosphorus-doped silicon film has been predominantly produced, which is formed adding a phosphorus compound gas as a dopant to a deposition gas in situ. In the phosphorus-doped film deposition process, the dopant is injected during deposition in the LPCVD method, which may solve the problem in the conventional phosphorus diffusion method. In addition, the deposited film may be activated by heating to achieve a much lower electrical resistance than that by a conventional phosphorus diffusion method. Thus, a phosphorus-doped silicon film formation is an essential process for manufacturing a recent advanced semiconductor device.
In general, a phosphorus-doped silicon film is deposited on one or more wafers by injecting SiH.sub.4 and PH.sub.3 gases into a CVD reactor at 500 to 600.degree. C. in which the wafers are placed. During the process, in a region more distant from an injector for the deposition gas, the gas is more consumed and PH.sub.3 gas originally in a less amount than SiH.sub.4 gas is more deficient, which may cause the proportion of PH.sub.3 gas in the deposition gas (partial pressure) to be reduced or uneven. Thus, it may lead to a problem that a phosphorus level in the film varies depending on the place of the wafer in the reactor. Therefore, in addition to a main injector, one or more PH.sub.3 injectors are generally provided in a reactor to make the phosphorus level even all over the reactor.
Such a CVD reactor, however, has the following problem. In a conventional process, at the end of a deposition process, all deposition gas injections are immediately stopped by setting a flow rate of each mass flow controller for the deposition gas to zero. A SiH.sub.4 /PH.sub.3 supply line for a main injector will stop its deposition gas supply with substantially no delay because of its large flow rate. In contrast, a supply line for a PH.sub.3 injector, where the flow rate is low, is long and difficult to be well-controlled in its flow rate. It may, therefore, take a time to completely discharge the remaining PH.sub.3 in the line or the mass flow controller, leading to flowing of the PH.sub.3 into the reactor. That is, while supply of SiH.sub.4 from the main injector is stopped, a small amount of PH.sub.3 from the additional PH.sub.3 injectors is continuously supplied into the reactor.
Due to lack of SiH.sub.4 supply, the unreacted PH.sub.3 which has flown into the reactor in such a manner adheres to the surfaces of the phosphorus-doped silicon films on the wafers, without being incorporated into the films, and a dummy wafer as well as on the inner wall of the reactor (FIG. 5(a)). At the end of the deposition process, the reactor is opened in the atmosphere for a next batch of deposition. If the reactor is opened in the presence of unreacted PH.sub.3 on the surfaces, the adhered PH.sub.3 may react with moisture in the air to produce phosphorus compounds such as phosphoric acid and P.sub.2 O.sub.5 (FIG. 5(b)). Since these phosphorus compounds are sublimable, they may be diffused out during the next batch of deposition in the reactor to adhere to surfaces of unprocessed wafers.
In deposition of a phosphorus-doped silicon film, if there is a surface region of an unprocessed substrate to which phosphorus compounds such as phosphorus oxides adhere, i.e., a region having a locally high level of phosphorus, the region tends to cause crystallization. Basically, phosphorus-doped silicon is deposited through growing of amorphous silicon. The growth rate is, therefore, very slow, i.e., 20 to 30 .ANG./min (ca.530.degree. C.), while in the region where crystallization has occurred the growth rate may be about twice. Thus, at the end of deposition, it may cause various problems such as anomalous growth and foreign materials including particles. FIG. 6 schematically shows an example of film formation generating a foreign matter, in which 61 is an Si substrate, 62 is an interlayer insulating film, 63 is a phosphorus-doped silicon film, 64 is a phosphorus compound and 65 is a foreign matter. When a stand-by time in the atmosphere until a next batch (waiting time for a reactor) is long, the unreacted PH.sub.3 may react with moisture sufficiently to sharply increase the number of foreign matters depending on the stand-by time. FIG. 4 is a graph showing the average number of foreign matters in relation to a stand-by time under the deposition conditions described later. Generation of such foreign matters may prominently reduce a production yield and an operating ratio due to necessity of washing the reactor, as well as may deteriorate properties and reliability of a product.